Download Simulation and Hardware Implementation of DC

Simulation and Hardware Implementation of DC-DC
Converter for Interfacing Energy Storage
Mr. S. D. Deshmukh1,
Dr. S. W. Mohod2,
PRMIT Amravati.
[email protected]
PRMIT Amravati,
1
Abstract- This paper presents simulation and hardware
implementation of maximum power point tracking
(MPPT) algorithm (IncCond) used in solar array power
systems with direct control method using PIC
controller. The proposed circuit integrates different
renewable energy sources as well as energy storage.
This integration will increase the reliability and
utilization of the overall system. Moreover, the
integration of energy storage solves the problem of the
slow response of renewable energy. The hardware
includes PIC controller, Optoisolator & Inverter
section. The circuit interfaces four power ports: two
sources, one bidirectional storage port, and one isolated
load port. Contributions are made in several aspects of
the whole system, including converter design, system
simulation, controller programming, and experimental
setup. The resultant system is capable of tracking MPPs
accurately and rapidly without steady-state oscillation,
and also, its dynamic performance is satisfactory.
MATLAB and Simulink were employed for simulation
studies. The proposed system was developed and tested
successfully. Experimental results indicate the
feasibility and improved functionality of the system.
Index Term- DC–DC converter, Hybrid distributed generation
system (HDGS), Photovoltaic (PV), Maximum Power Point
Tracking (MPPT),
1. INTRODUCTION
N
ow a days, the energy generated from clean, efficient,
and environmentally friendly sources has become one
of the major challenges for engineers and scientists [1].
Among all renewable energy sources, solar and wind power
systems attract more attention because they provide
excellent opportunity to generate electricity while
greenhouse emissions are reduced [1]–[3]. It is also
gratifying to lose reliance on conventional electricity
generated by burning coal and natural gas. Regarding the
endless aspect of solar and wind energy, it is worth saying
that these energy is a unique prospective solution for
energy crisis. However, despite all the aforementioned
advantages of solar power systems, they do not present
desirable efficiency [4], [5]. The efficiency of solar cells
depends on many factors such as temperature, insolation,
spectral characteristics of sunlight, dirt, shadow, and so on.
[email protected]
Changes in insolation on panels due to fast climatic
changes such as cloudy weather and increase in ambient
temperature can reduce the photovoltaic (PV) array output
power. In addressing the poor efficiency of PV systems,
some methods are proposed, among which is a new concept
called “maximum power point tracking” (MPPT). All
MPPT methods follow the same goal which is maximizing
the PV array output power by tracking the maximum power
on every operating condition.
The hardware proposes a new four-port-integrated
dc/dc converter, which is suitable for various renewable
energy harvesting applications. An application interfacing
hybrid photovoltaic (PV) and wind sources, one
bidirectional battery port, and an isolated output port is
given as a design example. It can achieve maximum powerpoint tracking (MPPT) for both PV and wind power
simultaneously or individually, while maintaining a
regulated output voltage.
1.1. MPPT Methods
There are large number of algorithms that are able to
track MPPs. Some of them are simple, such as those based
on voltage and current feedback, and some are more
complicated, such as perturbation and observation (P&O)
or the incremental conductance (IncCond) method. They
also vary in complexity, sensor requirement, speed of
convergence, cost, range of operation, popularity, ability to
detect multiple local maxima, and their applications [8]–
[10]. Having a curious look at the recommended methods,
hill climbing and P&O [11]–[16] are the algorithms that
were in the center of consideration because of their
simplicity and ease of implementation. Hill climbing [14],
[17] is perturbation in the duty ratio of the power converter,
and the P&O method [15], [18] is perturbation in the
operating voltage of the PV array. However, the P&O
algorithm cannot compare the array terminal voltage with
the actual MPP voltage, since the change in power is only
considered to be a result of the array terminal voltage
perturbation. As a result, they are not accurate enough
because they perform steady-state oscillations, which conse
quently waste the energy [8]. By minimizing the
perturbation step size, oscillation can be reduced, but a
smaller perturbation size slows down the speed of tracking
MPPs. Thus, there are some disadvantages with these
methods, where they fail under rapidly changing
atmospheric conditions [19].The IncCond method is the
one which overrides over the aforementioned drawbacks. In
this method, the array terminal voltage is always adjusted
according to the MPP voltage. It is based on the
incremental and instantaneous conductance of the PV
module [6], [19], [22].
configurations is a proper converter to be employed in
designing the MPPT. Hence, in the circuit we employed
IncCond algorithm in PIC controller by programming. The
output is studied and it’s quite satisfactory. For changing
voltage and current separate pots are used which help to
change duty cycle of pulses of PIC controller.
Fig 2. Cuk converter as MPPT.
Fig. 1 Basic Idea of IncCond method on a P-V curve
Fig. 1 shows that the slope of the PV array power curve is
zero at the MPP, increasing on the left of the MPP and
decreasing on the right-hand side of the MPP. The basic
equations of this method are as follows [24]:
IdI
=−-----,
dVV
=
dII
-----,
dVV
dII
= -----,
dVV
Fig. 2 shows the implementation of cuk converter with
IncCond algorithm using PIC controller.
at MPP ------------- (1)
left of MPP ------------- (2)
Fig. 3.1MPPT Output
right of MPP ------------- (3)
where I and V are the PV array output current and voltage,
respectively. The left-hand side of the equations represents
the IncCond of the PV module, and the right-hand side
represents the instantaneous conductance. From (1)–(3), it
is obvious that when the ratio of change in the output
conductance is equal to the negative output conductance,
the solar array will operate at the MPP.
Considering above facts proper selection of
converter is necessary. Both Cuk and buck–boost
converters provide the opportunity to have either higher or
lower output voltage compared with the input voltage.
Although the buck–boost configurations is cheaper than the
Cuk one, some disadvantages, such as discontinuous input
current, high peak currents in power components, and poor
transient response, make it less efficient. On the other
hand, the Cuk converter has low switching losses and the
highest efficiency among no isolated dc–dc converters. It
can also provide a better output-current characteristic due
to the inductor on the output stage. Thus, the Cuk
Fig. 3.2 MPPT Output
Fig 3.1 & 3.2 shows change in MPPT output duty cycle
with respect to change in input voltage & current.
2. HYBRID DISTRIBUTED
GENERATION SYSTEM
This system integrates different kinds of renewable
energy sources to increase the overall stability and
utilization [13]. A secure supply based on only one
renewable energy source requires substantial energy
storage. The reason is that the renewable energy is not
available during some period of time. For example, the
photovoltaic cells produce zero power during the night or
little power during the cloudy day. Wind energy may not be
available if the wind speed is under certain speed, under
which the wind turbine cannot extract energy from the
wind. If the system has to provide the power to load during
the time when renewable energy source produces little
power, the only way is to use the power from the energy
storage.
If two or even more different renewable energy
sources are integrated, the storage capacity requirement
would be greatly reduced, since the fluctuations of these
two renewable energy sources may have a statistical
tendency to compensate each other. For example, solar
power is high during the day, while the wind power is high
at night. Combining these two sources will approximately
produce a constant power to the load during the entire day.
fewer power devices, which means the cost of the power
converter will be lower than that of the conventional one.
Also, the conversion steps are minimized, resulting in
higher efficiency. Due to the presence of the transformer in
some circuits, electric isolation is available, which is
important for safety. With the turn ratio of the transformer
in certain topologies, it will be more efficient to integrate
different renewable energy sources of different voltage
levels. Finally, there is a central controller. The controller
not only controls the individual switches, but also manages
3. MULTI-PORT CONVERTER
4. HARDWARE IMPLEMENTATION.
There are two ways to integrate different renewable
energy sources and energy storage. The conventional way
is to use a common DC bus. Separated DC-DC power
converters are used to connect different renewable energy
sources to the DC bus. However, there are several
disadvantages for this structure. First, there are more power
electronic devices, resulting in higher cost. For each
renewable energy source, there has to be a DC-DC
converter, which connects the renewable energy source to
the DC bus. Also, there are more conversion steps. The
power is transformed from DC to DC, then from DC to AC
if the load is AC load.
Another way of integration is by using multi-port
power converters. The multi-port power converter will
connect all renewable energy sources and energy storage.
Some ports are bi-directional if they are connected with
energy storage, while some are unidirectional if they are
connected with energy source [14]. Fig. 4 shows one of the
applications of the multi-port DC-DC power converter.
This converter integrates fuel cell, photovoltaic cells,
energy storage and the load. If the load is AC, an extra
inverter is needed to convert the DC power into the AC
power.
In the design of PWM inverter, single or three phase,
the entire process of design is made into small modules.
The functionality of each section are tested separately. The
following section gives the details of each module of the
project and explains each module in the design. The PWM
approach is used for switching of inverter through the PIC
microcontroller.
Wind
Multiport
DC-DC
Power
Convert
er
L
Inverter
O
A
Energy
D
Storage
PV
Cell
Fig.4 Multi-port DC-DC power converter
In this multi-port DC-DC power converter, there will be
the whole system. the controller can control the switch to
perform the ”Maximum Power Point (MPP)” tracking for
the photovoltaic cell such that the output power is
maximum at any operating points The central controller
will enhance the overall performance of the system.
Fig. 5 Hardware Implementation
Fig. 5 is the block diagram that describes the
hardware development for controller circuit. The PWM
signal generated from PIC16F877A microcontroller is fed
opto-coupler and gate drives the inverter. The switching
frequency used in this project is 50Hz. The software
development includes facility for suitable switching pulses
with the use of the variable frequency and variable duty
cycle. It is desired to control the inverter with proper
switching purposes. The digital implementation usually
achieved with a timer based card inside microcontroller.
Modulation technique that is used in this project.
PIC generates the PWM pulses for MOSFET Bridge
which drives the gate of MOSFET. These pulses are
generated by microcontroller with respect to the changes in
duty cycle of MPPT. These pulses are then fed to
optoisolator. For single phase inverter four PWM pulses
and three phase inverter Six pulse are required to drive the
bridge of MOSFET. These pulses then drive the MOSFET
Bridge. By varying the TON and TOFF, the RMS output
voltage can be varied. Thus by varying duty cycle,
modulation index is varied.
4.1 PWM generation
Pulse-width Modulation is achieved with the help of a
square wave whose duty cycle is changed to get a varying
voltage output as a result of average value of waveform. A
mathematical explanation of this is given below.
destination may be at very different voltage levels. In such
situations the link between the two must be an isolated one,
to protect the subsystem from over voltage damage. The
isolation is required between control circuit and power
circuit in power electronics application. Three optical
isolators are used for providing the isolation between the
control circuit and power circuit. For isolation an optocoupler TLP 250 is used. TLP 250 provides an isolation
voltage of 2500Vrms [11].
Figure 5.1 Principal of PWM.
Consider a square wave shown in the figure above. Ton is
the time for which the output is high and Toff is time for
which output is low. Let Ttotal be time period of the wave
such that,
T total = Ton + Toff
Duty cycle of a square wave is defined as
Ton
Ton
D= ----------------- = ---------Ton + Toff
T total
The output voltage varies with duty cycle as...
Vout = D × Vin
Ton
Vout = ---------T total
Ton
Vout = --------- ×
Vin
Ttotal
So from the final equation the output voltage can be
directly varied by varying the Ton value.
If Ton is 0, Vout is also 0.
If Ton is Ttotal then Vout is Vin or say maximum.
Fig. 6.2 Optocoupler output
4.3 Inverter
In pulse width modulated (PWM) inverters, the input DC
voltage is essentially constant in magnitude and the AC
output voltage has controlled magnitude and frequency.
Therefore the inverter must control the magnitude and the
frequency of the output voltage. This is achieved by PWM
of the inverter switches and hence such inverters are called
PWM inverters. For square-wave inverters, the input DC
voltage is controlled in order to adjust the magnitude of the
output AC voltage. Therefore the inverter has to control
only the frequency of the output voltage. The output AC
voltage has a waveform similar to a square-wave. In single
phase inverter with voltage cancellation, it is possible to
control the magnitude and the frequency of the inverter
output voltage with a constant DC input voltage for a
different switch mode that is not pulse width modulated.
The inverter output voltage waveform is similar to square
wave.
Fig. 6.1 PIC Output
4.2 Optocoupler
In many situations the signals and data need to be
transferred from one subsystem to another within a piece of
electronics equipment without making a direct ohmic
electrical connection. This is because the source and
Fig. 6.3 Inverter output
the two variable voltage source resembling PV source and
wind source, a battery source and the Cuk converter. The
three different sources are modeled using electrical
characteristics to provide the output current and voltage of
the individual module. To realize ac voltage from wind
source to dc a rectifier diode is used. The provided current
and voltage are fed to the converter and the controller
simultaneously. The PI control loop is eliminated, and the
duty cycle is adjusted directly in the algorithm. To
compensate the lack of PI controller in the proposed
system, a small marginal error of 0.002 is allowed.
To test the system operation, each signal source
supplies voltages to the MOSFET drain terminal while a
ramp signal applied to gate terminal to produce PWM
signal. A three winding linear transformer is used to
integrate all the three inputs from different source. A
Fig. 6.4 Output at Load
Fig. 7 Hardware setup
5. SIMULATION
The diagram of the closed-loop system designed in
MATLAB and Simulink is shown in Fig. 8, which includes
universal bridge is used which acts as three-phase power
converter that consists of up to six power switches
connected in a bridge configuration.
Finally, an integrated output of all three supply
converted in single input using universal bridge. This input
is used to drive cuk converter. In feedback loop a set point
of 50v is given. The input voltage compares with this set
voltage and a constant output of 50 v is produced
irrespective of input voltage. Fig 8.1, 8.2,8.3 shows the
simulink
result
of
the
circuit.
Fig.8 Simulation Model
REFERENCES
Fig. 8.1 PWM pulse
Fig.8.2 Variation in V & I of input sources
Fig. 11(c) Constant voltage output
6. CONCLUSION
The Paper presented implementation of dc-dc converter
capable of four dc ports; two input source ports, a bidirectional
storage port, and a isolated loading port. This four port
converter is suitable for renewable energy systems, where the
energy storage is required while allowing tight load regulation.
For the Hybrid PV and wind system, the structure is able to
achieve maximum power harvesting, meanwhile maintaining a
regulated output voltage. The circuit operation of this converter
& its control system is experimentally verified. In this paper a
fixed size MPPT with control was employed with the help of
PIC controller. From the result acquired during the hardware
experiments, it was confirmed that, the implementation of
MPPT achieve an acceptable efficiency level of the PV
modules & track the maximum power.
The simulink model configuring for three modules PV,
wing and battery source shows in integrated form to
produce a constant output voltage using cuk converter
irrespective of input voltage values.
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